A power converter is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a dc input voltage into a dc output voltage. A power converter generally includes a controller to manage an internal operation thereof by controlling a conduction period of a power switch employed therein. Generally, the controller is coupled between an input and output of the power converter in a feedback loop.
The controller typically measures an output characteristic (e.g., an output voltage or an output current, or a combination of an output voltage and an output current) of the power converter and modifies a duty cycle of the power switch. The duty cycle of the power switch is a ratio represented by a conduction period of the switch to a switching period thereof. Thus, if a switch conducts for half of the switching period, the duty cycle for the switch would be 0.5 (or 50 percent). Additionally, as the needs for a load coupled to an output of the power converter dynamically change, (e.g., as a computational load on a microprocessor changes or an incandescent lamp or electromechanical device such as a motor is turned on), the controller is typically configured to dynamically increase or decrease the duty cycle of the power switch to maintain an output characteristic, such as an output voltage, at a desired value.
In an exemplary application, a power converter has the capability to convert an unregulated input voltage supplied by an input voltage source, such as 12 volts supplied by an internal power bus in a data processor, or a higher unregulated voltage, such as 36 volts supplied by an automotive battery in an advanced vehicular application, to a lower, regulated, output voltage, such as 2.5 volts to power an integrated circuit, or 12 volts to power an incandescent lamp or an electronic circuit.
To provide the voltage conversion and regulation functions, a power converter includes power switches such as metal-oxide semiconductor field-effect transistors (“MOSFETs”) that are coupled to the input voltage source and periodically switch a reactive circuit element such as an inductor to the voltage source at a switching frequency that may be on the order of several hundred kilohertz or higher.
In conventional power converter designs, it is necessary to provide a level of protection in the power converter for overcurrent, overvoltage, and fault conditions for sensitive internal components such as a power MOSFET, as well as a control strategy to regulate the output characteristic. Substantial effort is often expended in product manufacture to combine MOSFET fabrication technologies with additional features to provide protection mechanisms. For certain protection mechanisms, e.g., for overcurrent protection, additional circuit elements such as a current-sensing resistor are included in the power converter circuit which may add a recognized expense to the end product, particularly for a level of accuracy that may be necessary for such added circuit elements. A sensing element such as an operational amplifier must also be included in the circuit with supporting components to sense, for example, a voltage across the added circuit element. Such sensing elements and supporting circuitry add cost to an end product.
Despite inclusion of such protective mechanisms in power converters and in semiconductor switching arrangements, unsatisfactory system performance and protection often result using conventional design approaches. For example, a circuit providing an overcurrent limit for a power converter or a semiconductor switch coupled to an incandescent lamp often limits current to the lamp during its turn “on.” An “off” incandescent lamp can present a resistance to a power converter or a switch that can be less than one-tenth its resistance after its filament is heated to a normal operating temperature. Accordingly, the initial load current flowing to an incandescent load can exceed the steady-state load current by a factor of ten or more. Similar start-up issues are encountered with switching on other loads such as electromechanical devices, e.g., motors and relays. A current-limiting circuit in the power converter or associated with the switch generally limits load current to a level that is usually only modestly greater than the rated load current of the current source, for example, to a current level that may be 20% greater than the rated load current. A current limiting circuit that temporarily shuts down the electrical power source unnecessarily produces interrupted output current pulses that intermittently heat the lamp filament or initiate the operation of the electromechanical device. A current-limiting circuit that tolerates an acceptable level of current overload for the brief period of time, e.g., to heat the lamp filament would provide an improved current limiting arrangement, particularly if a substantial delay would otherwise be imposed on the electrical power source before it is restarted after an interruption.
The inverse situation can result when a power converter operates in a compromised environment, for example, an environment wherein an environmental temperature is higher than a maximum rated environmental temperature. An ordinary current-limiting circuit might not provide a shutdown function when it is required, possibly resulting in an immediate or a delayed power converter or semiconductor switch failure.
Thus, either an unnecessary shutdown or a failure to shutdown a power converter or a semiconductor switch can have untoward consequences for the operation of an electronic system, for example, an electronic system including a microprocessor. Accordingly, there is a need for an improved approach for a protective function that avoids the limitations of conventional protective design approaches, thereby providing improved system reliability and performance as well as reduced system cost.